Geotechnical Challenges of a New RCC Dam on the Oregon Coast

The City of Newport on the central Oregon coast receives all its water supply from a pair of earthen embankment dams that are over 50 years old and built at a time before the seismic hazard of the Cascadia Subduction Zone (CSZ) was identified. Both dams have been found insufficient to hold up to seismic loading of a CSZ earthquake or even a smaller event. A single replacement roller compacted concrete (RCC) dam has been proposed to replace both high hazard embankment dams that will increase storage capacity and withstand seismic loading. Through several investigations and evaluations, the geologic conditions have proven a challenge due to low strength bedrock, poor rock mass characteristics and a large landslide mapped in the right abutment. This expanded abstract focuses on the geotechnical challenges in the right abutment foundation and is not intended to provide in depth details on the foundation characteristics or other aspects of geotechnical design. The project is currently in progress with several evaluations yet to be performed to inform design decisions.